1. Field of the Invention
The present invention relates to a vacuum pump designed to detect a rotor temperature based on a temperature-dependent change in permeability (magnetic permeability) of a magnetic material, and optionally to control the rotor temperature according to the detection result.
2. Description of the Related Art
In a turbo-molecular pump for use in a semiconductor manufacturing apparatus and others, as gas to be evacuated by the turbo-molecular pump has a larger flow rate and/or molecular weight, a rotor temperature becomes higher due to heat generation associated with increased motor-driving power and frictional heat caused by discharge of the gas. Moreover, when gas having a lower thermal conductivity is evacuated, the rotor temperature will become higher. Generally, the rotor temperature becomes higher along with increases in a rotor speed, a flow rate, pressure and/or temperature of evacuation gas, and an air temperature around the pump.
A rotor of the turbo-molecular pump is rotated at a high speed, and thereby a large tensile stress acts thereon due to centrifugal force. Therefore, the rotor is typically made of an aluminum alloy excellent in specific strength. In contrast to this advantage, the aluminum alloy has a relatively low allowable temperature (about 110 to 140° C.) to creep deformation. Thus, it is necessary to constantly monitor a rotor temperature during operation of the pump so as to keep the rotor temperature at the allowable temperature or less.
For this purpose, there has been a technique of detecting a rotor temperature in a non-contact manner by utilizing a phenomenon that a permeability of a magnetic material is largely changed at a Curie temperature thereof (see, for example, Japanese Patent Laid-Open Publication No. 7-5051). Specifically, a ring-shaped ferromagnetic member is attached on an outer peripheral surface of a rotor, and a change in permeability of the ferromagnetic member at a Curie temperature thereof is detected by a coil.
Typically, in a process of detecting a rotor temperature, a signal level from a coil is compared with a threshold as a reference value to determine whether the rotor temperature increases beyond a Curie temperature, based on whether the signal level falls below the threshold. However, although a permeability is largely changed at the Curie point, a difference between a permeability at normal temperature and a permeability at a temperature beyond the Curie point is not so large as compared with the permeability change at the Curie point. Thus, the threshold has to be set within a narrow range to cause difficulty in accurately detecting the rotor temperature.